Under UK law, if you receive money in error, you have to have a good faith belief that the money is actually yours. To claim that you thought that the bank was deliberately choosing to give you more money than you asked for (rather than it simply being an error) stretches credulity a little too much.

See, for example, a family who were jailed for exactly this: http://www.guardian.co.uk/uk/2003/jan/16/rebeccaallison2

Citations welcome to this being different in other countries.